I found these charts to be pretty inaccurate. I used it just before getting a drop checker and had a PH of 6.6 and a KH of 6.5 telling me I had over 70 ppm of Co2.

I then put in the drop checker and it read a slightly dark green which is about 20ppm of Co2. I turned up the Co2 a little bit and got it at 30 - 40 (lighter green) and boom the plants started pearling like crazy. Other tests have indicated this chart not reading out how my drop checker is reading. I trust my drop checker over this personally.